1. Field of the Invention
The present invention relates to methods of inspection usable, for example, in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is necessary to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various inspection tools. One form of inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle. Principal component analysis is a known method used to obtain, for example, focus, dose and optionally contrast information from scatterometry data without the need for a computationally-intensive reconstruction of the target being measured. This is achieved by printing a calibration array of targets on a test substrate at different dose, focus and optionally contrast values. Scatterometry measurements are then performed on each target in the array. The measurement results are then decomposed into a set of orthogonal basis functions—known as the principal component functions, which depend on the target pattern used and coefficient values, known as the principal component values. Statistical techniques can then be used to establish a relationship between the nominal focus, dose and optionally contrast values used to print the targets and the principal component values. To derive focus, dose and optionally contrast values for a target printed in a production target, it is only necessary to make a scatterometry measurement, determine the principal component values and apply the derived relationship. Further details of this technique can be found in U.S. Patent Application Publication 2005/0185174 A1, which is hereby incorporated in its entirety by reference.
The relationship between the principal component values and focus, dose and contrast values is dependent on the structure underlying the target. Thus, the calibration array must be printed and measured in each layer in which targets are to be measured in each manufacturing process, it is not possible to reuse the relationship derived from one layer in one process in a different layer or process.
A limitation of the principal component analysis technique is that unintended variations in the underlying structure, e.g. layer thickness variations, between the calibration substrate and the substrate being measured can lead to significant errors.